The development of novel herbicide resistance in plants offers significant production and economic advantages. Rice production is frequently restricted by the prevalence of a weedy relative of rice that flourishes in commercial rice fields. The weed is commonly called "red rice," and belongs to the same species as cultivated rice (Oryza sativa L.). The genetic similarity of red rice and commercial rice has made herbicidal control of red rice difficult. The herbicides Ordram (molinate: S-ethyl hexahydro-1-H-azepine-1-carbothioate) and Bolero (thiobencarb: S-[(4-chlorophenyl)methyl] diethylcarbamothioate) offer partial suppression of red rice, but no herbicide that actually controls red rice can currently be used in rice fields because of the simultaneous sensitivity of commercial rice to such herbicides. The development of a mutant commercial rice that is resistant to a herbicide effective on red rice will greatly increase the ability to control red rice infestations.
Rice producers in the southern United States typically rotate rice crops with soybeans to help control red rice infestations. While this rotation is not usually desirable economically, it is frequently necessary because no herbicide is currently available to control red rice infestations selectively in commercial rice crops. During the soybean rotation, the producer has a broad range of available herbicides that may be used on red rice, so that rice may again be grown the following year. United States rice producers can lose $200-$300 per acre per year growing soybeans instead of rice, a potential loss affecting about 2.5 million acres annually. Additional losses in the United States estimated at $50 million per year ream the lower price paid by mills for grain shipments contaminated with red rice. Total economic losses due to red rice in southern United States rice production are estimated to be $500 to $750 million a year.
Rice producers typically use the herbicides propanil (trade name Stam) or molinate (trade name Ordram) to control weeds in rice production. Propanil has no residual activity. Molinate is toxic to fish. Neither of these herbicides controls red rice. Imazethapyr ((.+-.)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]- 5-ethyl-3-pyridinecarboxylic acid) offers an environmentally acceptable alternative to molinate, has the residual weed control activity that propanil lacks, and is a very effective herbicide on red rice. Imazethapyr also offers excellent control of other weeds important in rice production, including barnyardgrass. Barnyardgrass is a major weed in rice production, and is currently controlled with propanil or molinate. However, there are reports that barnyardgrass is developing resistance to propanil.
The total potential market for rice varieties that are resistant to a herbicide that can control red rice is about 5.3 million acres in the United States, and the market outside the United States is potentially much larger. World rice production occupies about 350 million acres. Red rice is a serious weed pest in rice production in the United States, Brazil, Australia, Spain, and in most other rice-producing countries. Herbicides that inhibit the enzyme acetohydroxyacid synthase would offer a number of advantages over currently available herbicides if they could be used in commercial rice production. Potential advantages include long residual activity against weeds, effective control of the more important weeds in rice production, including red rice, and relative environmental acceptability.
U.S. Pat. No. 4,761,373 describes the development of mutant herbicide-resistant maize plants through exposing tissue cultures to herbicide. The mutant maize plants were said to have an altered enzyme, namely acetohydroxyacid synthase, which conferred resistance to certain imidazolinone and sulfonamide herbicides.
Lee et al., "The Molecular Basis of Sulfonylurea Herbicide Resistance in Tobacco," The EMBO J., vol. 7, no. 5, pp. 1241-1248 (1988), describe the isolation and characterization from Nicotiana tabacum of mutant genes specifying herbicide resistant forms of acetolactate synthase (also known as acetohydroxyacid synthase), and the reintroduction of those genes into sensitive lines of tobacco.
Saxena et al., "Herbicide Resistance in Datura innoxia," Plant Physiol., vol. 86, pp. 863-867 (1988) describe several Datura innoxia lines resistant to sulfonylurea herbicides, some of which were also found to be cross-resistant to imidazolinone herbicides.
Mazur et al., "Isolation and Characterization of Plant Genes Coding for Acetolactate Synthase, the Target Enzyme for Two Classes of Herbicides," Plant Physiol. vol. 85, pp. 1110-1117 (1987), discuss investigations into the degree of homology among acetolactate synthases from different species.
Reference is also made to commonly-assigned U.S. patent application Ser. No. 07/657,429, filed Feb. 19, 1991, disclosing transformed plants with genetically engineered imidazolinone resistance, conferred through a gene cloned from a plant such as a mutated Arabidopsis thaliana. See also a related paper, Sathasivan et al., "Nucleotide Sequence of a Mutant Acetolactate Synthase Gene from an Imidazolinone-resistant Arabidopsis thaliana var. Columbia," Nucleic Acids Research vol. 18, no. 8, p. 2188 (1990).
Examples of herbicide-resistant AHAS enzymes in plants other than rice are disclosed in U.S. Pat. No. 5,013,659; K. Newhouse et al., "Mutations in corn (Zea mays L.) Conferring Resistance to Imidazolinone Herbicides," Theor. Appl. Genet., vol. 83, pp. 65-70 (1991); K. Sathasivan et al., "Molecular Basis of Imidazolinone Herbicide Resistance in Arabidopsis thaliana var Columbia," Plant Physiol. vol. 97, pp. 1044-1050 (1991); B. Miki et al., "Transformation of Brassica napus canola cultivars with Arabidopsis thaliana Acetohydroxyacid Synthase Genes and Analysis of Herbicide Resistance," Theor. Appl. Genet., vol. 80, pp. 449-458 (1990); P. Wiersma et al., "Isolation, Expression and Phylogenetic Inheritance of an Acetolactate Synthase Gene from Brassica napus," Mol. Gen. Genet., vol. 219, pp. 413-420 (1989); and J. Odell et al., "Comparison of Increased Expression of Wild-Type and Herbicide-Resistant Acetolactate Synthase Genes in Transgenic Plants, and Indication of Postranscriptional Limitation on Enzyme Activity," Plant Physiol., vol. 94, pp. 1647-1654 (1990).
S. Sebastian et al., "Soybean Mutants with Increased Tolerance for Sulfonylurea Herbicides," Crop. Sci., vol. 27, pp. 948-952 (1987) discloses soybean mutants resistant to sulfonylurea herbicides through a mechanism other than an altered form of the AHAS enzyme.
K. Shimamoto et al., "Fertile Transgenic Rice Plants Regenerated from Transformed Protoplasts," Nature, vol. 338, pp. 274-276 (1989) discloses a genetic transformation protocol in which electroporation of protoplasts was used to transform a gene encoding .beta.-glucuronidase into rice.
T. Terakawa et al., "Rice Mutant Resistant to the Herbicide Bensulfuron Methyl (BSM) by in vitro Selection," Japan. J. Breed., vol. 42, pp. 267-275 (1992) discloses a rice mutant resistant to a sulfonylurea herbicide, derived by selective pressure on callus tissue culture. Resistance was attributed to a mutant AHAS enzyme.
Following are publications by the inventor (or the inventor and other authors) concerning research on herbicide-resistant rice varieties. These publications are T. Croughan et al., "Rice and Wheat Improvement through Biotechnology," 84th Annual Research Report, Rice Research Station, 1992, pp. 100-103 (1993); T. Croughan et al., "Rice and Wheat Improvement through Biotechnology," 85th Annual Research Report, Rice Research Station, 1993, pp. 116-156 (1994); T. Croughan, "Application of Tissue Culture Techniques to the Development of Herbicide Resistant Rice," Louisiana Agriculture, vol. 37, no. 3, pp. 25-26 (1994); and T. Croughan et al., "Rice Improvement through Biotechnology," 86th Annual Research Report, Rice Research Station, 1994, pp. 461-482 (September 1995). (Note that the Annual Research Reports of the Rice Research Station are published in the year after the calendar year for which activities are reported. For example, the 84th Annual Research Report, Rice Research Station, 1992, summarizing research conducted in 1992, was published in 1993.)
Following are two commonly-assigned United States patent applications concerning herbicide resistant rice, also filed by the present inventor: Ser. No. 08/345,213, filed Nov. 28, 1994, now U.S. Pat. No. 5,545,822; and Ser. No. 08/628,031, filed Apr. 4, 1996.